This invention relates to the recovery of silver from exhausted or spent photgraphic processing solutions such as photographic fix.
Several processes are known for recovering silver from photographic waste products. One technique involves a process where the waste products are burned under controlled conditions such that silver can be recovered from the ash. U.S. Pat. Nos. 2,944,886 and 3,671,222 relate to such techniques.
Other processes for the recovery of silver utilize chemical reactions. In U.S. Pat. Nos. 2,131,045; 3,547,987,986; and 3,960,550, silver sludge is produced from the waste material through chemical reaction and silver is subsequently recovered from the sludge. U.S. Pat. No. 3,832,453 discloses a process where silver is recovered from photographic waste solutions by mixing and reacting a first flowing stream of fixing solution with a second flowing stream of an aqueous sulfide solution to precipitate silver particles. The precipitated particles are then separated from the residual solution which can be reused.
A third method of reclaiming precious metals, such as silver, from solution is through the use of filters. In U.S. Pat. No. 2,643,108, silver is precipitated from a silver salt solution by zinc plates which are held within a fabric bag. In operation, the bag is placed into a tank containing hypo solution and the normal circulation of the liquid solution in the tank causes the liquid to flow laterally through the bag, between the lead plates, such that the silver salts held in solution are precipitated within the bag.
Other U.S. patents, such as U.S. Pat. Nos. 3,630,505; 3,744,995; and 4,240,617 deal with silver recovery units having fluid permeable metallic filter elements. In these devices, the silver-containing solution reacts with the metal of the filter element so that the silver is deposited therein. The metal filter elements are made of a metal above silver in the electromotive force series, such as steel wool or galvanized window screen. U.S. Pat. No. 4,213,600 shows a cylindrical container in which a plurality of fluid permeable layers of a chemical reduction agent are disposed in interleaved relation with a plurality of fluid permeable layers of a non-reactive filter material. A galvanic effect is used to precipitate silver in this apparatus.
All of the above-described prior art methods of recovering metals from solution require complex apparatus and/or the use of chemical reactions to separate desired components from undesired ones. It would be advantageous to have a simple, low cost and effective apparatus to recover precious metals, e.g. silver, which are suspended in a liquid, e.g. spent photographic developing solutions. Such an apparatus should be lightweight, of small size so that it can sit harmlessly on a counter top, and of fool-proof construction.
The need for such an apparatus, particularly for use by small scale processors of photographic films is heightened by the ever-increasing value of silver. Dentists and doctors, for example, currently dispose of photographic developing solutions which contain valuable amounts of silver. Costly prior art systems for silver recovery, designed particularly for use by large scale processors of photographic films, are impractical for small scale operations. Such prior art systems are not only too expensive for use by amateur photographers, doctors and dentists; they are also too complicated to use.
An ideal system for use by such small scale users would not require any additional steps over those already being used in the developing process. Such a system would not require the user to add chemicals, make frequent filter changes, or conduct preventative maintenance.
This invention relates to such apparatus.